There are a few normal requirements to be able to apply for a US card.

US Social Security number

USD income

Credit record

US mailing address

There are probably others too.

In short, it’s hard, but not impossible – however you do essentially need to be living and working over there.

One related option is an Amex international account transfer. This allows you to transfer your whole Amex account to another country. You won’t pick up bonus points and you need to prove you have a US bank account and mailing address etc to pay the card, but it removes some of the need for a credit record so is good for expats moving over to the US.